This invention relates to reflectance spectroscopy of surfaces which are fully or partially covered with films, coatings, or contamination, and in particular to an accessory for use therein.
Fourier transform infrared (FTIR) spectroscopy is a powerful and widely used analytical technique. When it is applied to solid surfaces, three different techniques of reflectance spectroscopy can be used: specular reflectance, diffuse reflectance, and reflection-absorption spectroscopy (see P. R. Griffiths and J. A. de Haseth, Fourier Transform Infrared Spectroscopy, Wiley-Interscience, 1986, chapter 5.III). Specular reflectance is used on flat, clean surfaces and spectra are readily obtained from comparatively small samples at incidence angles of about 15-75.degree.. Diffuse reflectance spectra are commonly obtained from matte or rough surfaces, or from the surface of powdered samples, often using specially designed spectrometers or accessories. The third type of reflectance spectrum, commonly called a reflection-absorption spectrum, is fairly easily obtained from surfaces coated with thick films in the range of 0.2-20 .mu.m. Simple reflectance spectra obtained from such surfaces are in effect transmission spectra obtained while the spectrometer radiation passes through the film before and after being reflected from the surface. However, in the case of extremely thin layers, such as monomolecular layers, the path length through the film may be too short to afford a usable spectrum. In practice, layers less than about 10 .mu.m thick cannot be successfully detected or characterized using conventional specular or diffuse reflectance geometries. This problem has been recognized, and the method commonly known as "grazing angle" FTIR has been developed. The method is explained in detail by Griffiths and de Haseth (op. cit., p. 189). Briefly, the thickness d (in .mu.m) of a film which will give a reflectance-absorption spectrum of the same intensity as the transmittance spectrum of a 10 .mu.m film is given by the expression: EQU 2d=10 cos.alpha.
where .alpha. is the angle of incidence (measured from the perpendicular). As .alpha. approaches the grazing angle, the effective path length of the radiation through the film is increased. It is therefore advantageous to arrange for the radiation to strike the surface under examination at an angle approaching the grazing angle. Spectrometer accessories which allow for variable angles of approach by the radiation, such as those proposed by Milosevic and Harrick (U.S. Pat. No. 5,048,970) are available for this purpose.
It is a disadvantage of these prior-art accessories that they are placed inside the sample compartment of the spectrometer, so that samples have to be brought to the spectrometer, and must be of limited size. Methods are known for utilizing optical fiber cables to enable in-situ spectroscopic sampling of a wide range of different types of samples (see, for example, U.S. Pat. Nos. 5,170,056 by Berard, Burger, Melling and Moser and U.S. Pat. No. 5,754,722 by Melling).
It is an object of the invention to combine the spectroscopic advantages of the grazing-angle method with the ease and convenience of using a fiber-optic cable to provide a spectroscopic means for characterizing and quantifying thin films on reflective surfaces such as metals, without the inconvenience and impracticality of removing samples to the spectrometer.